The JSTL (JavaServer Pages Standard Tag Library) fn tag library provides a set of functions for manipulating strings, collections, and other objects in JSP (JavaServer Pages) pages. One of the functions in the fn tag library is the fn:replace function, which is used to replace all occurrences of a substring in a string with another substring.

The fn:replace function takes three arguments:

1. The string to search in
2. The substring to search for
3. The substring to replace the occurrences with

The function returns a new string with all occurrences of the search substring replaced with the replacement substring. The syntax of the function is as follows:

${fn:replace(string, substringToSearchFor, substringToReplaceWith)}

where string is the string to search in, substringToSearchFor is the substring to search for, and substringToReplaceWith is the substring to replace occurrences of substringToSearchFor with.

For example, to replace all occurrences of the substring "foo" in the string "hello foo world" with the substring "bar", you would use the following expression:

${fn:replace('hello foo world', 'foo', 'bar')}

This expression returns the string "hello bar world", where all occurrences of the substring "foo" have been replaced with the substring "bar".

You can also use variables instead of literals for the arguments. For example, if you have a variable text containing a string, you can replace all occurrences of a substring in it using the following expression:

${fn:replace(text, 'foo', 'bar')}

This expression returns a new string with all occurrences of the substring "foo" replaced with the substring "bar" in the string stored in the text variable.

Note that the fn tag library must be imported at the beginning of the JSP page using the following tag:

<%@ taglib prefix="fn" uri="http://java.sun.com/jsp/jstl/functions" %>

This imports the fn tag library and assigns it a prefix of fn.